Standard practice in the disposal of toxic wastes or low-level radioactive wastes requires incorporating the wastes into a non-leaching or low-leaching wasteform and burying the wasteform in a permitted site. The usual strategies for producing a wasteform involve blending the solid (sludge or salt) and liquid waste as a slurry in a dry mixture that contains a filler and a cementitious product such as portland cement or fly ash and lime. Because cement-based wasteforms have some porosity and may leach salts, an alternate strategy has involved mixing dry waste salts into a thermosetting polymer such as asphalt or polyethylene. Neither technique is totally satisfactory when used alone.
Portland cement-based wasteforms present problems due to the interaction of the waste and the cement, which can prevent the wasteform from solidifying; cause the wasteform to solidify too quickly; or cause the wasteform to lose strength after initially hardening. The problems related to interferences with the cementing reactions are so great that with a typical sulfate or nitrate waste, the salt loading is typically less than 15 percent of the mass of the wasteform. If detergents or surfactants are present the loading may have to be even lower. Although the wasteforms made as cement-based composites can be manufactured as strong, coherent masses, they still show porosity that is typically in the range observed in concrete. Soluble salts will be leached from these wasteforms if they are exposed to groundwater.
Asphalt-based wasteforms have been widely used for disposal of low-level radioactive wastes, especially those containing soluble salts. This is because the wasteform can be manufactured with dried salts and because leaching rates are generally very low due to the hydrophobic nature of the material. When asphalt, polyethylene, or other polymers are used to encapsulate waste, problems arise from the combustibility of organic materials when mixed with strong oxidizers, such as chlorates and nitrates. Radiation from the enclosed wastes can degrade the polymer and generate hydrogen gas. As the hydrogen gas accumulates it pressurizes the containers holding the asphalt thus creating an explosive potential.
In past applications of solidification in waste disposal, the accepted approach to forming a solid from a waste has been to mix the waste as a slurry, solution or dry solid with an organic or inorganic cementing or encapsulating medium and to create conditions that would allow the cementing medium to harden. This technology has been documented for both radioactive wastes (Dlouhy, Zdenek, 1982. Disposal of Radioactive Wastes, Elsevier Scientific Publ. New York, and conventional industrial wastes (U.S. EPA 1980. Guide to the Disposal of Chemically Stabilized and Solidified Waste, SW-872, U.S. EPA, Washington, D.C.).
Waste composites have been fabricated using organic polymers and portland cement-based mortars; but in prior applications, the mortar and the polymers have been mixed together to let the organic polymer fill the void space in the hardened mortar. For example, SYNCRETE, a polymer-portland cement mixture developed for waste disposal involves mixing water, a polymer emulsion, portland cement, wastes and a catalyst to produce a hardened block containing wastes (Cohen, S., P. Crouzet, 1986. "SYNCRETE: A highly efficient polymer cement embedding matrix for waste processing". Waste Management '86, Waste Isolation in the US.Proceedings of the Symposium, March 2-6, 1986, Tucson, AZ, pp 583-588). The polymer interpenetrating the matrix produced by the hydration of the calcium silicate in the portland cement is thought to produce the exceptional strength observed in this composite. Unfortunately, the polymer does not isolate the waste and the components in the waste can prevent the polymerization of the polymer and also stop the hardening reactions that occur in the portland cement. The present invention differs significantly from the SYNCRETE approach because the new system isolates the waste in a polymer and coats the polymer-waste mixture before the waste is added to the portland cement-based mortar. The new technology represents a significant improvement over the prior art because it prevents any of the components in the waste from interfering with the setting reactions that occur in the portland cement hydration.
Dlouhy (1982) describes another wasteform that is manufactured by mixing the waste with portland cement to form a weak block that is then strengthened by impregnating the block with organic polymer. Unfortunately, this technique requires that the block be vacuum dried at 165 degrees C. and soaked in the heated polymer. In the example cited, the block is held in liquid styrene at 85 degrees C. for 40 hours. (Dlouhy, Zdenek 1982 . Disposal of Radioactive Wastes, Elsevier Scientific Publ. Co. New York, NY p. 138). The new technique does not involve polymer impregnation and can proceed faster with the advantage that the wastes will not weaken the portland cement matrix, and no final impregnation will be required. The polymer impregnation also has a significant disadvantage in that it is difficult to insure that the polymer in the waste block has polymerized without breaking or coring the block. Both of these steps destroy the integrity of the waste block. In the new system the condition of the waste-polymer composite can be determined by inspection prior to the addition of the pellets of composite to the portland cement-based matrix.
An alternate method for encapsulating wastes in polyethylene was proposed by Lubowitz et. al., in 1977. In the method discussed by Lubowitz et. al., dried wastes were stirred into an acetone solution of modified 1,2-polybutadiene for five minutes and then the waste/polymer mixture was allowed to set for two hours. The polymer-impregnated particles are placed in a mold and subjected to mechanical pressure and heated to between 120 and 200 degrees C. to produce fusion. A polyethylene jacket approximately 3.5 mm (1/2 in.) thick is fused over the solid block. The proposed disposal method would use 800 to 1000 lb. blocks (Lubowitz, H. R., R. L. Denham, and G. A. Zakrzewski, 1977 Development of a Polymeric Cementing and Encapsulating Process for Managing Hazardous Wastes. U.S. EPA Publ. EPA-600/2-77-045, U.S. EPA, Cincinnati, Ohio,. This technique has serious drawbacks in that all of the organic polymers and solvents used are flammable. If an oxidizer (such as chlorate or nitrate) is mixed with acetone and polybutadiene and heated, care must be taken to avoid a fire. Also the polyethylene jacket can deform (squeeze thin) under pressure and can be punctured. Damage in handling and stacking can compromise the integrity of the outer polyethylene jacket. In contrast, the new technique avoids these problems by using only thermoplastic media and techniques that have been routinely used with oxidizing salts and embeds coated pellets of organic polymer in a portland-cement mortar mix that will not flow or deform plasticly and poses no risk of fire. Furthermore, the organic polymer is distributed through an inorganic matrix that separates the pellets and destroys the physical continuity of the combustible material.